Welcome back !! Hope it wasn’t something I said .
Ok you’re doing a lot inthe sump. Remember you’re still going to be changing water. Going with juveniles look at daily 50-90% water changes. There’s nothing cheap about juveniles. Tons more work and no guarantee you get what you ordered. My advice buy the biggest you can afford. These will be strong healthy discus. You’re saving all the money you’d be spending on food and water changes. Yes you’ll need to feed those little guys 6x a day!!
As to the sump you don’t need the floss in the socks. Just clean the socks with every water change. The matten filter are sponge right? I’d add one big enough that the water has to go through it. The k2 they say is great. Th Swiss stuff is sponge? And ceramics in the end is fine though I’m starting to think they’re worthless.